Singleton class - by overloading "new" operator

[ishwarDhumal]

Singleton class - is a class of which only one object exists in memory at a time.
We can create singleton class using :

• A static method or

• By overloading "new" operator for that class.

Here is a simple example of creating singleton class by overloading "new" operator for that class.

#include<iostream.h>
#include<conio.h>
#include<alloc.h>
 
class iSingleton
{
 static iSingleton *sptr;    //this pointer points to object of the class
 public:
    iSingleton()    //public constructor
    {
       sptr=this;
    }
 
 void *operator new(size_t);    //overload new operator for class which is to be made iSingleton.
};
 
 iSingleton *iSingleton::sptr=NULL;    //initialize pointer to NULL
 
void *iSingleton::operator new(size_t s)
{
  if(sptr!=NULL)    //if already one object is created return reference to same object
     return sptr;
  else
     return malloc(s);    //else allocate memory for one (first) object
}
 
int main()
{
 clrscr();
 iSingleton *sptr1=new iSingleton;      //first object created
 iSingleton *sptr2=new iSingleton;      //second object created
 
 if(sptr1==sptr2)
      cout<<"\nGiven class is a Singleton class.";
 else
      cout<<"\nGiven class is not a Singleton class.";
 
 getch();
 return 0;
}
 

[kiseitai2]

Wow, very interesting and useful. I'm currently working on my first videogame (it's a for fun project) and was wrapping my head around how could I use the least amount of memory when creating objects like floor blocks in a 2D game. This information could prove useful. Say, if I have a block on the screen that is a singleton class in memory, does that mean that if I want its texture in multiple locations on the screen I cant simply change the rectangle in the class because it is only one object in memory? In other words, can I replicate the texture without creating copies of the the class? I'm thinking of a platform type of game. Also, what situations are singleton classes most often used in the professional world?

[ishwarDhumal]

I think Singleton class is useful in situation where only one instance of the class is to be reused throughout the application.

• It provides global access to the single instance of the class and

• It guarantees that no more than one instance of that type can ever be created.

I don't know about 2D game programming so can't say about it..

[vitthaldevkar]

gr8, very interesting and useful. this code is very optimize for singleton design pattern.

[weaknessforcats]

Did anyone read: http://bytes.com/topic/c/insights/65...erns-singleton?

[sumitdhyani]

Nice thought, just a suggestion:
It works fine till the objects are created on heap using operator new.

Since the default constructor is still public, multiple number of stack objects can be created by writing:
iSingleton ob1;
iSingleton ob2;//.. and so on

What you can do to avoid this is to declare the destructor private. That way the code won't even compile if someone dares to declare automatic( or even static ) objects( since declaring an automatic objects implicitly means calling the destructor at some later point).

The only way left to create objects then will be to use operator new.

[weaknessforcats]

It's not that there is one singleton object. You cannot prevent someone creating an object either on the stack or the heap.

The key is that there is a universal point of access where only one object is referenced. That is the singleton object.

To find the correct object, you use the Instance() method of the object. So if you have 25 objects of the Singleton class you will find that the Instance() method of these objects always returns the same address. That address is the official singleton.

[ishwarDhumal]

Thanks SumitDhyani for your reply.